So every 7 or 8 years we need our batteries replaced. That presents big problems and big costs. How to get around that?

In “I Sing the Body Electric,” poet Walt Whitman waxed lyrically about the “action and power” of “beautiful, curious, breathing, laughing flesh.” More than 150 years later, MIT materials scientist and engineer Canan Dagdeviren and colleagues are giving new meaning to Whitman’s poem with a device that can generate electricity from the way it distorts in response to the beating of the heart.

Despite tremendous technological advances, a key drawback of most wearable and implantable devices is their batteries, whose limited capacities restrict their long-term use. The last thing you want to do when a pacemaker runs out of power is to open up a patient just for battery replacement.

The solution may rest inside the human body — rich in energy in its chemical, thermal and mechanical forms.

The bellows-like motions that a person makes while breathing, for example, can generate 0.83 watts of power; the heat from a body, up to 4.8 watts; and the motions of the arms, up to 60 watts. That’s not nothing when you consider that a pacemaker needs just 50 millionths of a watt to last for seven years, a hearing aid needs a thousandth of a watt for five days, a smartphone requires one watt for five hours.

Short version is as long as you are alive, your body will power your device. Full article (possibly behind a paywall) here.

"The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge" S Hawking

One energy-harvesting strategy involves converting energy from vibrations, pressure and other mechanical stresses into electrical energy.

A different energy-harvesting approach uses thermoelectric materials to convert body heat to electricity.

Scientists have also sought to use the same effect behind everyday static electricity to power devices.

Another strategy relies on devices — known as biofuel cells — that generate electricity via chemical reactions between enzymes and fuel molecules in, or released from, the body, such as glucose in blood, or lactate in sweat.

I have often said they will someday find a method to create a hydro electric plant within the arteries using the flow of blood to power a small "water wheel" creating power! Unfortunately, I wasn't smart enough to do it and get rich!